In optical networks, for example dense wavelength division multiplex (DWDM) systems, a plurality of channels are multiplexed. The term channel is used to refer to a wavelength channel, also known as an optical frequency channel. In many such systems, a pilot tone is used for optical performance monitoring (OPM). The pilot tone is a small and low-frequency modulation (e.g. kHz to MHz) applied to a high-speed optical channel. A pilot tone provides an in-band ancillary channel for performance monitoring. Each channel is modulated with a different pilot tone (frequency). Accordingly, the power of a particular pilot tone can be useful for indicating the power of the high speed optical channel in a wavelength division multiplexed (WDM) system.
Some prior art systems modulate pilot tone signals with a data signal used to carry channel-characterizing information, e.g. modulation format, baud rate, source/destination. Modulating the pilot tone with channel information is useful, as the pilot tones of all the DWDM channels can be detected using a low speed detector without needing to demultiplex the DWDM signal. However, conventional pilot tone data modulation typically uses on/off keying (OOK) modulation, which means there is no power in the pilot tone signal for each zero bit of data. This means the presence of the high speed optical signal can no longer be detected for each zero bit of pilot tone data. This can be particularly problematic when there is a number of successive zero data bits in the pilot tone data signal.
Accordingly there is a need for an improved pilot tone method and system.